Problem statement

We'd like to assign true fitness values to antigen viruses for use in forecasting model validation. However, antigen doesn't encode fitness directly into viruses.

Proposal

As John suggested, a good way to do this using the existing antigen codebase would be to assign fitness scores to the tips of the tree. These fitness values would be the average risk of infection the viruses poses to the total (or portion) of the host population.

A lot of the code to do this already exists -- will just have to write some new functions to handle the above.